Define and give the classsification of infection

Q. Define and give the classsification of infection.

Ans : Infection and immunity involve interaction between the animal bes (host) and the infecting microorganism. Based on their relationship to their hosts, microorgan-isms can be classified as saprophytes (from Greek sapros decayed; andphyton plant) and parasites.
   Saprophytes are freeliving microbes that subsist on dead or decaying organic matter. They are found in soil and water and play an important role in the degradation of organic materials in nature. They are generally inca-pable of multiplying on living tissues and therefore are of little relevance in infectious disease. Exceptionally, however, some saprophytes like B.subtilis may infect devitalised hosts whose natural resistance is greatly reduced (opportunistic infection).
   Parasites are microbes that can establish themselves and multiply in hosts. Parasitic microbes may be either path-ogens (from Greek pathos suffering, and gen produce, that is, diseaseproducing) or commensals (from Latin com with; and mensa table, that is living together).
   Pathogens are microorganisms that are capable of producing disease in the host. Commensal microbes live in complete harmony with the host without caus-ing any damage to it. The normal bacterial flora of the body consist largely of commensals. Many commensals behave as facultative pathogens in that they can pro-duce disease when the host resistance is lowered.
It is necessary to distinguish between the term ‘infection’ and ‘infectious disease’. The lodgement and multiplication of a parasite in or on the tissues of a host constitutes infection. It does not invariably result in disease. In fact, disease is but a rare consequence of infection, which is a common natural event.

Classification of Infections :
Infections may be classified in various ways.
a. Initial infection with a parasite in a host is termed primary infection. Subsequent infections by the same parasite in the host are termed reinfections.

b. When a new parasite sets up an infection in a host whose resistance is lowered by a preexist-ing infectious disease, this is termed secondary infection.

C. Focal infection (more appropriately focal sepsis) indicates a condition where, due to infection or sep-sis at localised sites such as the appendix or tonsils, generalised effects are produced.

d. When in a patient already suffering from a disease a new infection is set up from another host or another external source, it is termed crossinfection. Crossinfections occurring in hospitals are called nosocomial infections (from Greek nosocomion or hospital).

e.The term iatrogenic infection refers to physicianinduced infections resulting from investigative, therapeutic or other procedures. Depending on whether the source of infection is within or outside the host’s own body, infec-tions are classified as endogenous or exogenous, respectively. Based on the clinical effects of infections, they may be classified into different varieties.

f. Inapparent infection is one where the clinical effects are not apparent. The term subclinical infection is often used as a synonym.

g. Atypical infection is one in which the typical or characteristic clinical manifestations of the particu-lar infectious disease are not present.

h. Latent infection is one in which some parasites, following infection, may remain in the tissues in a latent or hidden form, proliferating and producing clinical disease when the host resistance is lowered.

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